58 THE CRAYFISH [CH. II 



also in the skeleton and hypodermis is due to 'tetron- 

 erythrin/ a pigment which also has a strong affinity for 

 oxygen. The amceboid corpuscles are active in devouring 

 bacteria and other injurious organisms that have obtained 

 entry into the system. 



The heart, which receives the blood through its valved 

 ostia from the pericardial sinus, is stimulated to contract 

 by the entrance of the oxygenated blood 1 . The hinder 

 portion contracts first and then the anterior, by which 

 time the hind section has begun to dilate. Rise of tem- 

 perature causes the rate of beating to increase. The 

 average rate is from sixty-five to eighty-five beats per 

 minute. The heart is under the control of the nervous 

 system, and has a double nerve-supply, (1) a median un- 

 paired nerve from the stomatogastric ganglion, (2) nerves 

 from the thoracic ganglia of the ventral chain: stimulation 

 of the former quickens, but of the latter retards the heart- 

 beat. The heart may be revived to activity after the beats 

 have ceased, and may be maintained beating for some time 

 by the application of weak acetic acid. 



The excretion of waste nitrogenous matter is effected 

 by the pair of green glands, whose discharging apertures are 

 in the bases of the large antennae. Uric acid and other 

 nitrogenous compounds akin to urea have been detected 

 in these glands (Griffiths). 



Nervous System and Special Senses. The physiology 

 of the nervous systems of Invertebrates has been investi- 

 gated but little. It is however known that in the cray- 



1 Plateau, vide Nature, xix. p. 470, 1879. 



